The goal of this project is to elucidate the mechanisms that regulate the P family of transposable elements in Drosophila melanogaster. In the absence of regulation, the members of the P family actively transpose, causing a syndrome of germ-line abnormalities called hybrid dysgenesis; the traits of this syndrome include an elevated mutation rate, frequent chromosome breakage, segregation distortion and sterility. When the P family is regulated, the incidence or severity of these traits is reduced. Genetic analyses have indicated that regulatory ability varies among Drosophila strains and that it depends on the P elements themselves. Although the prevalent view is that regulation is mediated by P-encoded polypeptides that repress the synthesis or activity of the P transposase, this project will emphasize a different hypothesis -- that P activity is repressed by antisense P RNA. There are four specific aims: (1) Sense and antisense P element constructs driven by inducible promoters will be tested in vivo to determine if they can repress P element activity. This will allow an assessment of the feasibility and robustness of the polypeptide and antisense repression hypotheses. (2) Cloned DNA containing known repressor P elements will be mutated in vitro and then tested in vivo to determine if repression is mediated by a polypeptide or an antisense RNA. (3) Drosophila stocks collected from populations around the world will be screened for P elements inserted in an antisense orientation downstream of retrotransposon promoters. These insertions will then be isolated genetically and tested for repression ability. (4) Assorted Drosophila stocks, including stocks with individual isolated P elements and stocks that have been transformed with P element constructs, will be analyzed to determine if there is a correlation between the presence of antisense P RNA and the ability to repress P activity. This research has the potential to document a regulatory mechanism that may apply to other transposon families and to certain types of eukaryotic viruses, including some that cause human disease. It may therefore open a new perspective on the behavior, regulation and evolution of these genetic parasites.